fix(build): use libgit2's bundled zlib so the server image builds on arm64 hosts

[stigi]

Summary

Building the server image on an arm64 host fails while linking libgit2 for the
amd64 target, before any Go code is compiled:

/rootfs-amd64/usr/lib/x86_64-linux-gnu/libz.a(deflate.o): relocation R_X86_64_PC32
  against symbol `z_errmsg' can not be used when making a shared object; recompile with -fPIC
/usr/lib/gcc-cross/x86_64-linux-gnu/12/../../../../x86_64-linux-gnu/bin/ld: final link failed: bad value
collect2: error: ld returned 1 exit status
make[2]: *** [src/libgit2/CMakeFiles/libgit2package.dir/build.make:476: libgit2.so.1.5.0] Error 1

backend/Dockerfile builds libgit2 with BUILD_SHARED_LIBS=ON, and cmake
resolves zlib to the static libz.a in the staged cross sysroot. That archive
is not built with -fPIC, so it cannot be linked into a shared object. Point
libgit2 at its own bundled zlib for both cross targets instead: it is compiled
as part of the shared library and is therefore position independent.

The failure is host-architecture dependent, which is why CI does not see it —
on an amd64 builder the amd64 target is native and picks up a usable zlib. On
Apple Silicon, which is where a lot of contributors work, docker build --platform linux/amd64 cannot produce the image at all.

Why not the alternatives

• Installing zlib1g-dev into the staged sysroots would pull the same static
  archive; the shared libz.so symlink is what is missing, and staging it
  correctly for both architectures is more moving parts than one cmake flag.
• CMAKE_POSITION_INDEPENDENT_CODE=ON does not help: the offending object is
  in a prebuilt Debian archive, not in anything this build compiles.
• Building only the target architecture would also work, but the loop over both
  architectures looks deliberate, so this change leaves it alone.

Does this close any open issues?

No issue filed — the reproduction and the error are above. Happy to open one if
you would prefer the bug tracked separately.

Screenshots

Not applicable; this is a build change.

Verification

On an Apple Silicon host (Docker Desktop, --platform linux/amd64):

• before: fails at Dockerfile:64, the libgit2 stage, with the relocation error above
• after: links libgit2.so, the stage completes, and the build proceeds through
  the Go plugin build to a working image

The resulting amd64 image has been running a DevLake instance, collecting from
a live data source, without issue.